Modernized Engine Wiring

OneOfMany said
Please correct me if I am wrong:
Watts = Volts x Amps

And I'll agree when you are right. Correct, Power = current x voltage (DC or instantaneous if AC).

OneOfMany said
A 12g wire supports 2400 watts. Therefore a 12g wire in a car can support 200amps.

Incorrect. Current is the only rating that matters for a wire. The power dissipated in the wire is (current)^2 * (resistance). This power must be dissipated as heat and conducted away. The later is different between home installation and appliances, so different current ratings. The voltage rating of the wire is for when the electrical insulation breaks down (arcs to ground).

At the risk of confusing people, you also see the power loss written as (V^2)/R, but V here is not the 120 V or 12 V max in the circuit, but the dV (voltage drop) across the wire itself. Of course, if you connect the wire directly from your battery to ground, dV = 12V, but don't do that. Walter White did that on Breaking Bad to a jerk in a BMW.